This invention relates to a cathode ray tube display system comprising a cathode ray display tube having means for producing an electron beam and directing the electron beam into a space between a planar array of elongate deflection electrodes and the input side of a laminated dynode channel electron multiplier, a screen disposed adjacent the output of the electron multiplier onto which the current multiplied beam from the multiplier is directed, and driving means connected to the electron beam producing and directing means and to the deflection electrodes for applying potentials thereto. The driving means is operable to provide an accelerating potential for the beam and to switch the potential applied to each of the deflection electrodes between a first predetermined potential V.sub.1 and a second predetermined potential V.sub.2 and to control the sequence in which each of the electrodes is switched so as to cause the electron beam entering the said space to be scanned across the input side of the multiplier.
A cathode ray display tube of the above kind is disclosed in British Patent Specification No. 2101396. The cathode ray tube described in this specification is generally flat and has an electron gun which directs a low energy electron beam parallel to the screen. A reversing lens at one end of the tube turns the beam through 180.degree. such that it travels in an opposite direction parallel to the screen towards the space between the planar array of deflection electrodes and the input surface of the channel electron multiplier, both of which are arranged parallel to the screen. The deflection electrodes comprise vertically spaced, horizontally elongate electrodes which are selectively energisable to establish an electrostatic field having a component normal to the screen which deflects the electron beam in that space towards the input side of the multiplier. By progressively energising the deflection electrodes in turn, the point of deflection of the electron beam in the longitudinal direction of that space is correspondingly progressively moved so as to effect frame scanning. The deflected electron beam undergoes current multiplication within the electron multiplier and is then accelerated towards the screen by means of an accelerating electrostatic field between the output side of the multiplier and the screen. In order to achieve orthogonal scanning of the beam over the input of the multiplier in raster fashion, the tube further includes elastrostatic deflectors arranged adjacent the final anode of the electron gun and prior to the reversing lens which deflect the beam in a plane parallel to the screen, thereby effecting line scanning. Although the specific description of the flat display tube in this specification refers to line scanning in the horizontal direction by the electrostatic deflectors and field scanning in the vertical direction by the planar array of deflection electrodes, the display tube may be rotated through 90.degree. so that the line scanning is vertical and the field scanning is horizontal. For convenience of description field/frame scanning will be taken to mean the lower of the two scanning speeds.
According to British Patent Specification No. 2101396, the display tube is operated by passing the initially accelerated electron beam produced by the gun through a field free space defined between a pair of planar electrodes situated on either side of the beam's path intermediate the gun and the reversing lens, which are held at 400 V. An electron beam acceleration voltage of 400 V is provided by applying 400 V to the gun's final anode. Line scan is effected by applying to the line deflectors a line scan waveform modulated at field frequency with a peak value of typically .+-.50 V. The input side of the electron multiplier is also held at 400 V whilst at the beginning of each frame scan the deflection electrodes are at 0 V but are brought up to 400 V in sequence so that the electron beam, having passed through the reversing lens, is initially deflected into the topmost apertures of the multiplier by the field created between the electrodes at 0 V and the 400 V input side of the multiplier. The electrodes are brought up to 400 V in turn so that a field free space is formed between those electrodes at 400 V and the input side of the multiplier and the point of deflection of the beam, determined by the next electrode in the group to be at 0 V, is progressively moved downwardly in the direction of the incoming beam. All the above specified voltages are with reference to the cathode potential of the electron gun at 0 V.
Whilst this known tube generally performs well, it has been found that visible Moiree patterning effects can be produced on the screen during operation which spoil the otherwise good quality picture obtained. Obviously these effects are irritating and distracting for the viewer and it is an object of the present invention to provide a display system which does not suffer in this respect or in which these effects are at least reduced to an acceptable level.